Meters providing for variable and selectable scale readout over a large range of parameters are well known in the prior art. These prior art meters are generally electrical measurement instruments and encompass meters limited to the measurement of a single parameter as well as meters capable of providing measurement of many parameters. Such prior art instruments have, however, presented a more or less cluttered appearance, causing the reader to momentarily confuse or misread the scales and thereby often affording the substantial opportunity for error in any individual reading or series of readings. Prior meter presentations have also tended to be inaccurate due to parallax error inherent in using a flat printed scale and due to the constant need to realign the zero point on the scale as scales are changed. In addition, many of these prior art meters tend to be bulky and less susceptible of portable hand-held operation due to complex mechanical linkages and the substantial amount of space occupied by the scale changing mechanism.
For example, the meter disclosed in U.S. Pat. No. 3,092,072 utilizes a single rotable circular plate on which is imprinted a sequence of numbers representing a range of parameters and is mechanically rotated by a rack and pinion arrangement to permit selected numerals to be sequentially visible through windows in the meter housing, masking other numerals. While permitting an arcuate readout, this meter has a limited number of ranges and scales which the operator may select from and is unsuitable for use in portable hand-held meters.
Among the various prior art meters, some provide for the simultaneous automatic selection of both the proper scale and the proper electrical circuit which operates the indicating movement. In general, selection is accomplished by activating a separate switch thus not affording direct coordination between the scale and the movement operating circuit. Those prior art devices which do provide for a simultaneous selection of both the proper scale and the corresponding movement operating circuit do not have the simplicity and rugged construction afforded by this invention. One example is the device disclosed in U.S. Pat. No. 3,829,775 requires the use of a servo-driven pointer and due to its use of a linear flat scale and its concomitant parallax error is incomptable for accurate measurements with a D'Arsonval movement. Furthermore, the complexity of the mechanical linkages and components to effect simultaneous selection makes this device unfeasible for low-cost manufacture, carefree maintenance and ease of hand-held operation. Another example is the device disclosed in U.S. Pat. No. 2,558,276 which requires a complex mechanical arrangement to convert the motion of the selector switch in one plate to move the scale carrying down in a perpendicular plane. Such an arrangement necessitates expensive precision assembly and is susceptible to malfunctions in the rugged environment of hand-held test instruments used in the field.